Delay lines having nondispersive width-shear mode propagation characteristics and method of making same

ABSTRACT

Thin guided waveform delay lines of this invention feature a maximum thickness in the order of two 1/2 wavelengths with full width transducers coextensive with the delay line thickness providing nondispersive width-shear mode propagation characteristics. A low cost mass production method of making a substantially identical series of such delay lines if also disclosed including steps of forming a composite preshaped block of acoustically conductive material with signal transducers affixed thereto and then cutting the composite block with its signal transducers into a plurality of substantially identical delay line segments. Completed delay line segments each can be housed in a casing having associated electrical circuit components of each delay line segment.

United States Patent [72] Inventors John F.Belford Westport; Donald E. Gibeau, Hazardville; John Ciriello, Canton, all of, Conn.

[2]] Appl. No. 763,199

[22] Filed Sept. 27, 1968 [45] Patented May 25, 1971 [73] Assignee Andersen Laboratories, Incorporated Bloomfield, Conn. Continuation-impart of application Ser. No. 736,675, June 13, 1968.

[54] DELAY LINES HAVING NONDISPERSIVE WIDTH- SHEAR MODE PROPAGATION CHARACTERISTICS AND METHOD OF MAKING SAME 9 Claims, 6 Drawing Figs.

[52] US. Cl 333/30,

[51] lnt.Cl H03h 9/16,

H03h 9/30 [50] Field of Search 29/2535; 333/30, 30 (M), 71, 72 [56] References Cited UNITED STATES PATENTS 2,505,515 4 1950 Arenberg 333/30 2,672,590 3/1954 McSkimin 333/30 2,752,662 7/1956 Crooks et al.. 29/2535 2,781,494 2/1957 Geoghegan... 333/30 2,864,013 12/1958 Wood 29/2535 May, Jr.; J. E., Guided Wave Ultrasonic Delay Lines, Chapt. 6, Physical Acoustics, Vol. 1, Part A, Edited by Mason; W. P., pp. 440 444, 1964 Anderson, Anderson Labs., lnc., Advertisement Solid Ultra-sonic Delay lines, Form #100 25C 453, Received in Patent Office 4-1954 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Wm. H. Punter Attorney-Prutzmah, Hayes, Kalb & Chilton ABSTRACT: Thin guided waveform delay lines of this invention feature a maximum thickness in the order of two A wavelengths with full width transducers coextensive with the delay line thickness providing nondispersive width-shear mode propagation characteristics. A low cost mass production method of making a substantially identical series of such delay lines if also disclosed including steps of forming a composite preshaped block of acoustically conductive material with signal transducers affixed thereto and then cutting the composite block with its signal transducers into a plurality of substantially identical delay line segments. Completed delay line segments each can be housed in a casing having associated electrical circuit components of each delay line segment.

PATENTEDHIIYZSISII 3.581247,

' SHEET 1 OF 2 FIG.

DIRECTION OF PARTICLE MOTION I21? 7 /4 f M M Z5 OIREcTION OF WAVE PROPAGATION Zfl F/G. a 31 /Z/ I x ,5 2/ Z4 INVENTORS JOHN F, BELFORD DONALD E. GIBEAU JOHN CIRIELLO ATTORNEYS PATENTEU was l9?! 3581.247

SHEET 2 [1F 2 PLATI N6 CLEAgNiNG HEATIING INVENTORS JOHN F. BELFORD DONALD E. GIBEAU JOHN CIRIELLO f 1 ATTORNEYS DELAY LINES HAVING NONDISPERSIVE WIDTH- SIIEAR MODE PROPAGATION CHARACTERISTICS AND METHOD OF MAKING SAME This application is a continuation-impart of applicants prior copending application Ser. No. 736,675 filed June 13, 1968, and assigned to the assignee of this invention.

This invention generally relates to delay lines and particularly concerns improved ultrasonic delay lines and a method of making them.

A principal object of this invention is to provide an improved method of very precisely making thin delay lines of a type utilizing two major reflecting surfaces for guided wave propagation in a low cost process featuring minimal tolerance requirements and significant labor saving techniques.

Another object of this invention is to provide improved delay lines particularly suited to be made by the above method for nondispersive width-shear mode propagation of symmetrical shear waves.

A further object of this invention is to provide such improved delay lines having full width transducer apertures to ensure guided wave propagation with a minimum of distortion and spurious content while at the same time effecting signal transmission with minimal transmission losses.

A still further object of this invention is to provide an improved delay line assembly having a rugged, sealed construction capable of furnishing reliable operation over an extended period of time under adverse environmental conditions.

Other objects will be in part obvious and in part pointed out more in detail hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties, and the relation of elements, which are exemplified in the following detailed disclosure.

In the drawings:

FIG. 1 is a plan view of a strip delay line;

FIG. 2 is an end view ofthe line of FIG. 1;

FIG. 3 is an enlarged side view of the line of FIG. 1;

FIG. 4 is an isometric view of a block of delay material suited to be sliced into individual delay lines;

FIG. 5 is a plan view, partly broken away and partly in section, showing a preferred embodiment of a delay line assembly constructed in accordance with this invention; and

FIG. 6 is a schematic diagram illustrating successive stcps of a manufacturing process according to this invention.

Referring now to the drawings in detail, FIG. 1 shows a delay line It) comprising a pair of transducers 12, I4 affixed to ultrasonic signal input and output facets 16, I8 ofa solid plate 20. The plate 20 is formed of acoustically suitable material conductive of ultrasonic waves such as quartz, alloys such as Invar, etc. The transducers I2, 14 are preferably formed of piezoelectric crystalline or polycrystalline material.

The delay line provides a signal transmission delay to an electrical signal applied to the transducer 12, which will be understood to be an input transducer arranged for connection to a suitable signal source, not shown. Ultrasonic energy entering the input facet 16 traverses the plate along a path, generally shown by line 21, e.g., in FIG. 3, to the output facet 18 of the plate 20. The ultrasonic energy is then impressed upon the output transducer 14 which reconverts ultrasonic energy into electrical pulses, providing a delayed signal to a suitable output circuit, not shown, connected to the output transducer 14.

While ultrasonic energy can be propagated in a delay medium by different types of elastic waves, shear waves will be considered for purposes of this invention. Shear waves are characterized by particle motion normal to the direction of wave propagation to effect a shear stress in the material. In addition, shear waves can be polarized and have a suitably low velocity to provide a required delay in a given structure.

It will be understood that transducers 12, 14 are poled parallel to the width dimension of the plate 20 for propagating symmetrical shear waves or so-called SH waves such that vibratory motion of the particles is normal to the direction of wave propagation (FIG. I) and parallel to major faces 22, 24 of the delay medium. I.e., the direction of the particle motion is perpendicular to both the longitudinal axis of the plate 20 and its minor faces 26, 28.

Upon any internal reflection at the major faces 22, 24, polarized SH waves are simply reflected at all angles of incidence since the particle motion is parallel to the major faces 22, 24. However, a diverging SH wave striking a reflective surface such as the minor faces 26, 28 of the plate 20 will undergo mode conversion from shear to compressional waves at all angles of incidence up to a critical. angle, which for most delay media is less than 45. Accordingly, the only mode conversion due to such divergence or beam spread will occur in the width dimension of the delay medium and even this mode conversion, which may be minimal due to the shortness of a particular delay line, can be eliminated if the angle of reflection is maintained greater than the critical angle of the delay medium.

Upon launching a pulse into the plate 20, undesirable spurious rays may arise in various ways to produce a series of pulses at the output transducer 14 due to different delay paths taken by the rays. If the pulse duration is greater than the difference in the time delay between different paths, undesired pulse overlap may occur to distort the received signal. Just as beam spread and the resulting unwanted mode conversion may give rise to undesirable spurious signals, the latter may also result from reflections at the output transducer facet 18 providing multiple or triple travel signals with resultant delays which are multiples of the desired main delay. In addition, spurious signals may arise by ultrasonic energy escaping from the intended path through side lobes of the transducer radiation pattern and from inaccurate construction of the plate 20 or faults in the delay material.

To minimize unwanted spurious signals, particularly those due to beam spread and side lobe radiation, conventional delay lines are commonly formed with a thickness greater than the acoustic beam or, as is well known, by avoiding reflection at one or both major faces 22, 24 by conventionally applying an absorbing coating to absorb such wave motion or by scattering the wave motion by deliberately roughening the faces, introducing yet another step into the normally time-consuming and expensive processes of making delay lines. Moreover, as the thickness of the line is increases, large blocks of material with large transducers significantly increase the cost, size and weight of the delay line, and dispersive modes are encountered. In dispersive modes, the phase velocity of each wave depends upon its own frequency and the wave form, as it travels, will change appreciably from that of the input pulse.

To provide a delay line particularly suited for low cost manufacture and assembly and which makes maximum utilization of the available energy without requiring close manufacturing tolerances or the provision of acoustic absorbing media to the boundary surfaces of the delay line, a waveguide form of delay medium is provided in accordance with this invention having nondispersive width-shear mode propagation characteristics.

More specifically, delay line 10 is in a waveguide form of thin plate delay media having a maximum thickness of two '6 wavelength which can be readily manufactured with the use of normal techniques to effect a delay line having acceptable frequency dispersion. In those applications for which very thin media are particularly suited, a line less than a half wavelength thick is preferred since only the nondispersive mode would exist whereby the delay is insensitive to changes in frequency. For a mathematical development of guided wave propagation of symmetrical shear modes in plates, an article by T. R. Meeker and A. H. Meitzler, in Physical Acoustics, Edited by W. P. Mason, Volume I, Part A, Academic Press,-l964, Pages 1 l9-l22, shows that the group velocity and the phase velocity are independent of frequency for the zeroth order mode which is the only nondispersive SH mode.

By the above-described construction of delay line I0, its major faces 22, 24 each in effect provide a mirror surface for concentrating all energy of the beam and guiding it continuously to the output transducer 14. Accordingly, side lobes of the transducer radiation pattern will strike the major faces 22, 24 at small angles, and the diverging and side lobe radiation will reach the output transducer 14 in substantially the same delay time as the rays passing parallel to the major faces 22, 24 such that the loss in signal transmission will be negligible.

In the specific illustrated embodiment, any possibility of beam spread is minimized in the width dimension of the line 10 by providing a line width of several wavelengths. The line 10 herein described is particularly suited for low frequency operation in the order of megacycles, over a temperature range of l5-55 C. and below, with a delay of about 64 microseconds and a delay time tolerance at 25 C. of about i 3 nanoseconds.

As the size of the transducers 12, I4 is reduced, the tendency of beam spreading is proportionately increased. To minimize any tendency of such beam spreading as well as multiple or triple travel due to reflection, e.g., at the transducer facet l8, spurious signals are further reduced by providing a full width transducer aperture wherein the transducers 12, 14 are coextensive with the thickness of the plate 20 at the transducer .facets l6, 18. Such full width apertures effect the further advantage of minimizing diffraction at the output end of the line 10. In contrast, a delay medium having a thickness of several wavelengths with full width transducers would accentuate unwanted dispersive modes and undesired dissipation of ultrasonic energy would occur.

To obtain satisfactory transmission of signals, with a minimum of distortion and spurious content, it is normally necessary to fabricate the delay line of material used as the delay medium very precisely, and to perform the bonding and fabrication of the transducers in a very exact manner. This is particularly true of delay lines utilizing reflecting boundaries. Frequently, the same boundary is used many times to reflect a signal, and any error in angle or finish may be effectively multiplied by the number of reflections. In addition, if very precise delay times are required, the distances between reflecting faces must be held very accurately. Typically in making conventional delay lines, angular tolerances are held to the order of seconds of arc, and linear tolerances to tenths of one thousandth of an inch. It is apparent, therefore, that considerable expense is normally involved in producing delay lines.

To significantly reduce costs in the mass production of high quality delay lines of a waveguide-type utilizing an SH wave and full width transducers, a block of acoustically conductive material is initially configurated and dimensioned in accordance with this invention to a preselected size and shape to provide precise angular and dimensional orientation between the transducer and reflecting facets, as determined by a desired signal delay. In FIG. 4, a polyhedron 30 is shown suitably dimensioned to provide a multiplicity of thin plates of a so-called 3 X 4 pattern ensuring a folded path designated by lines 31 wherein the angles of incidence on reflective minor surfaces such as at 33 are beyond the critical angle for mode conversion. In FIG. 6, another polyhedron 32 is shown for providing a suitable 2 X 5 pattern without danger of mode conversion. The method of making such delay lines will be described in connection with polyhedron 32.

Polyhedron 32 is provided with four mutually perpendicular side wall surfaces 34, 36, 38, 40 and a pair of converging end wall surfaces 42, 44 meeting at an apex or edge 45. The end wall surfaces 42, 44 are preselected as input and output facets of the block 32 and meet at a 90 angle while extending respectively at 45 angles with respect to a flat bottom wall surface 46. The block 32 is then cleaned in a well-known manner, e. g., with the use of alcohol and demineralized water.

A pair of signal transducer strips such as the one shown at 48 (48 in the polyhedron 30 shown in FIG. 4) are formed of suitable material such as piezoelectric crystalline or polycrystalline material and are carefully oriented in accordance with conventional techniques and appropriately dimensioned to fit onto the end wall surfaces 42, 44 of the block 32. The transducer strips 48 are then bonded or otherwise affixed in any suitable manner to the end wall surfaces 42,44 of the block 32, for example by first plating one or both sides of the strips 48 with a pure metal foil or film such as at 50 to provide a thin metallic film between the block 32 and the transducer strips 48. It will be understood that the above bonding method is set forth only by way of example and various other acceptable bonding methods may be used such as that described in US. Pat. No. 2,688,121 entitled Ultrasonic Delay Line and assigned to the assignee of this invention.

The composite block with its bonded transducer strips 48 may then be tested in a conventional manner to calculate its transmission time. Having determined a delay time measurement, the block as a whole may be adjusted to correct any minute deviations from a desired time delay, e.g., by precisely grinding the bottom wall surface 46 to reduce the size of the block along its major dimension and thereby reducing the effective length of the transmission path 51, shown by broken lines in FIG. 5.

After the block 32 has been reduced in size to effect any minor time delay adjustment, the composite block 32 may be secured in a suitable fixture of a wafer sawing machine, not shown, and then cut along spaced parallel planes normal to a common axis of the block 32 shown at X-X. A refined sawing operation can provide a multiple gang of blades (not shown) providing, e.g., approximately cuts in a single pass.

By virtue of the disclosed method of this invention, the cutting action determines in a single operation the size of each of a series of discrete delay line segments with its prepositioned transducers I12, 114 (112, 114' in the polyhedron 30 shown in FIG. 4) being coextensive with their mating transducer facets 116, 118 (116', 118' in the polyhedron 30) in the direction of the common axis X-X. In addition to ensuring that coextensive transducers are obtained, this method also assures that each individual delay line segment will be provided with interfaces automatically aligned in substantially identical relation to that of the others of the series of delay line segments. The finished solid polygonal delay plates 120 thus effect a multiple internal reflection delay path between input and output transducers 112, 114 preformed on each individual delay plate 120. I

As will be apparent to those skilled in the art, accumulative tilt error of each individual delay line segment, which is of crucial importance, is minimized by the initial grinding operation on the polyhedron 30. While the sawing operation may introduce minordeviations between minor faces 33 and the major faces of the plates caused by sawing polyhedron 30 along the broken lines 121 of FIG. 4, e.g., such deviations have no effect whatsoever on accumulative tilt error, and the manufacturing tolerances insofar as tilt is concerned are significantly less critical.

It will now be seen that a multiplicity of production steps are eliminated which were heretofore normally associated with the mass manufacture of delay lines. A multiplicity of delay lines can be formed by performing the precision fabrication just once on a large block in a minimal number of steps and without requiring any additional steps of applying absorbing media to the delay lines or otherwise providing irregular faces thereon. In meeting requirements of miniaturized applications, e.g., the composite structure is significantly easier to handle in obtaining close tolerances and critical dimensioning, while tending to promote increased uniformity, e.g., during the adjustment of the transmission delay time. Moreover, the predominant cost factor, that of skilled labor, is significantly reduced while greatly increased output is achieved, for precision transducer attachment need be performed just once for a multiplicity of delay lines by virtue of cutting up affixed transducers simultaneously with the delay medium. Moreover, if a better major surface finish is required it is only necessary to grind, lap or polish the sliced surfaces in multiple laps.

In accordance with another feature of this invention, suitable associated circuit components, such as a pair of tuning coils 52 and 54, are provided with each coil having suitable electrical leads 56, S8 and 60, 62 connected to a pair of pin terminals 64, 66 and 68, 70, respectively. The coils 52,54 with their respective leads and terminals are prepositioned in fixed relation within a case 72 formed of a suitable tough plastic which is resistant to warpage, extreme temperature changes, shock and moisture to provide a rugged and durable protective housing for each delay line 110. The latter need only be inserted through an open end 74 of the case 72 to bring the input and output transducers 112, 114 into engagement with a spring-type contact, preferably provided, such as at 76, on each terminal. The open end 74 of the case 72 through which the delay line 110 is inserted may then be sealed by a snap-in cover 78 which when secured additionally serves to maintain the delay line 110 in operative position. The case, if desired, may be shielded by a metallized finish and appropriate mounting pins, not shown, may also be provided to mount the case 72 to associated apparatus. The preferred embodiment of the case 72 is shown with a pair of access openings 80, 82 aligned with screw slots 84, 86 on the tuning coils 52, 54 for effecting any desired final adjustment of each tuning coil over a desired inductance range although the uniformity of delay lines produced by this method normally permits use of a fixed or preadjusted coil.

it will now be seen that finished delay line assemblies may be quickly and easily produced in accordance with the method of this invention to ensure close dimensioning and precision adjustment in a uniform high quality production operation while achieving increased output at significant cost savings.

As will be' apparent to persons skilled in the art, various modifications and adaptations of the structure abovedescribed will become readily apparent without departure from the spirit and scope of the invention, the scope of which is defined in the appended claims.

We claim: a

l. A method of making a multiplicity of substantially identical acoustical delay lines each having a pair of major faces respectively providing an internal reflection surface and full width transducers coextensive with the delay line thickness and exhibiting nondispersive width-shear mode propagation characteristics, the method comprising the steps of bonding signal transducing means directly to a peripheral surface of a block of acoustically conductive material having a predetermined size and shape, and then cutting the bonded block and signal transducing means along spaced parallel planes generally perpendicular to the peripheral surface to form a multiplicity of thin delay line plates having a waveguide form of a preselected thickness with each delay line plate having identical delay line segments made in accordance with the method of claim 1.

4. A method of making a plurality of substantially identical acoustical delay lines comprising the steps of first preforming a block of acoustically conductive material with a pair of flat signal input and output surfaces, then bonding signal transducing means to the signal input and output surfaces respectively, and then cutting the bonded block and signal transducing means along a plane generally perpendicular to the signal transducing means on both the input and output surfaces to form discrete delay line segments of a preselected size.

5. The method of claim 4 further including the step of adjusting delay time by reducing the size of the block after bonding the transducing means thereto but before the cutting step.

6. The method of claim 4 further including the steps of inserting a delay line segment into a case having terminal means prepositioned therein, and securing the delay line segment with its transducing means in contact with the terminal means of the case.

7. The method of claim 4 wherein the preforming step further includes configuring the block to the shape of a polyhedron, providing a multiple reflection internal transmission delay path between said signal input and output surfaces.

8. The method of claim 4 wherein the preforming step includes configuring the block .to the shape of a polyhedron with said pair of flat signal input and output surfaces being of elongated form, and wherein the bonding step comprises affixing a pair of elongated strips of signal transducing material, constituting said signal transducing means, to said pair of signal input and output surfaces.

9. The method of claim 8 wherein the preforming step further includes providing said pair of input and output surfaces in different planes respectively extending parallel to a common axis of the polyhedron, and wherein the cutting step comprises simultaneously severing the bonded block and transducing means along spaced parallel planes normal to said common axis of the polyhedron. 

1. A method of making a multiplicity of substantially identical acoustical delay lines each having a pair of major faces respectively providing an internal reflection surface and full width transducers coextensive with the delay line thickness and exhibiting nondispersive width-shear mode propagation characteristics, the method comprising the steps of bonding signal transducing means directly to a peripheral surface of a block of acoustically conductive material having a predetermined size and shape, and then cutting the bonded block and signal transducing means along spaced parallel planes generally perpendicular to the peripheral surface to form a multiplicity of thin delay line plates having a waveguide form of a preselected thickness with each delay line plate having signal transducing means secured thereon in prepositioned coextensive relation to its thickness.
 2. The method of claim 1 further including cutting the bonded block and signal transducing means into thin delay line plates having a waveguide form and a maximum thickness of two 1/2 wavelengths of the wave which is to be propagated.
 3. A delay line comprising one of a series of substantially identical delay line segments made in accordance with the method of claim
 1. 4. A method of making a plurality of substantially identical acoustical delay lines comprising the steps of first preforming a block of acoustically conductive material with a pair of flat signal input and output surfaces, then bonding signal transducing means to the signal input and output surfaces respectively, and then cutting the bonded block and signal transducing means along a plane generally perpendicular to the signal transducing means on both the input and output surfaces to form discrete delay line segments of a preselected size.
 5. The method of claim 4 further including the step of adjusting delay time by reducing the size of the block after bonding the transducing means thereto but before the cutting step.
 6. The method of claim 4 further including the steps of inserting a delay line segment into a case having terminal means prepositioned therein, and securing the delay line segment with its transducing means in contact with the terminal means of the case.
 7. The method of claim 4 wherein the preforming step further includes configuring the block to the shape of a polyhedron, providing a multiple reflection internal transmission delay path between said signal input and output surfaces.
 8. The method of claim 4 wherein the preforming step includes configuring the block to the shape of a polyhedron with said pair of flat signal input and output surfaces being of elongateD form, and wherein the bonding step comprises affixing a pair of elongated strips of signal transducing material, constituting said signal transducing means, to said pair of signal input and output surfaces.
 9. The method of claim 8 wherein the preforming step further includes providing said pair of input and output surfaces in different planes respectively extending parallel to a common axis of the polyhedron, and wherein the cutting step comprises simultaneously severing the bonded block and transducing means along spaced parallel planes normal to said common axis of the polyhedron. 